The present invention provides method and apparatus for a fiber strap termination. Specifically, the fiber strap termination area is formed from a rigid fiber reinforced composite, which allows the strap to be securely attached to a structural member.
Fiber straps are typically mounted at one end to a strap drum and at an opposite end to a movable structural member. The strap retracts and extends from the drum to move the structural member to a desired location. In one example, the strap drum is incorporated into a tilt rotor or tilt wing aircraft. This type of aircraft utilizes a pair of rotor systems which are pivotable such that the rotors may assume a vertical or horizontal orientation. In a horizontal orientation (i.e., horizontal rotor plane), the aircraft is capable of hovering flight, while in a vertical orientation (i.e., vertical rotor plane), the aircraft is propelled in the same manner as conventional propeller-driven fixed-wing aircraft. The strap drum is incorporated into a Variable Diameter Rotor (VDR) system. This system is used to change the outer diameter of the rotors depending upon the orientation of the rotor system. Typically, when the plane of the rotor is oriented horizontally, the rotor diameter is enlarged for improved hovering efficiency and, when oriented vertically, the rotor diameter is reduced for improved propulsive efficiency. Usually the strap drum is centrally mounted with respect to the rotor blades and includes one strap for each of the blades. One end of a strap is mounted to a movable portion of the blades while the opposite end is attached to the drum. The strap is extended from the drum to increase the diameter of the blades and is retracted to decrease the diameter of the blades.
In another example, the strap drum is incorporated into an elevator system. The elevator moves between different vertical levels in building to transport passengers and cargo to desired locations within the building. The strap drum is typically fixed relative to the elevator and one end of the strap is mounted to a movable elevator component. The strap extends and retracts from the drum to move the elevator component to a desired location.
One important aspect of the fiber strap is the termination portion. The termination portion is the end portion of the strap that is secured to the movable structural member, such as the rotor blade or the elevator component. Current methods for strap termination have resulted in decreased normal strength of the strap due to premature failures at the termination area. One method terminates the strap by looping the strap over a pin and then stitching the overlapped portions of the strap together. Another method consists of clamping the strap between metal plates that additionally require looping of the strap, and then fastening the strap to the structural member.
Both of these methods have experienced premature failures at the start of termination area. Additionally, because both methods required the strap to be looped, significant assembly time is required and increased packaging space is required for the terminated strap portion to be secured to the structural component.
Accordingly, it is desirable to provide strap termination method and apparatus that is easily assembled, has increased normal strength and is robust, and which can be utilized more effectively in areas with little available packaging space.
The drive strap that extends and retracts from a strap drum according to the present invention includes a strap member with an extended body portion made from a first material and a termination portion made from a second material having a higher rigidity than the first material.
In the preferred embodiment, the first material is a flat fiber rope and the second material is a rigid fiber-resin composite material. A transition region is preferably formed between the body portion and the termination portion and is made from a third material that has a higher rigidity than the first material and a lower rigidity than the second material. The third material is a flexible matrix-fiber composite.
Also in the preferred embodiment, the body portion is formed from layers of spanwise fibers and the termination portion is formed from alternating layers of spanwise fibers and angled fibers. This results in the termination portion having a greater thickness than the body portion. The orientation of the fibers in addition to the rigid fiber composite material increases the overall strength of the termination portion.
In one embodiment, the drive strap is used in a variable diameter rotor blade assembly that includes a rotor hub with multiple rotor blades each having an outboard blade section telescopically mounted to an inboard blade section. A reeling assembly includes the strap drum that extends and retracts the strap attached to each of the outboard blade sections to selectively change the diameter of the rotor blade assembly. The strap includes a body portion extending toward the drum and a termination portion attached to each of the outboard blade sections. The termination portion is made from a rigid fiber reinforced composite that has a higher rigidity than the body portion.
At least one fastener is used to secure the termination portion to the outer blade section. Preferably, the termination portion is significantly shorter than the body portion but is long enough to support the required number of fasteners.
The method for terminating the fiber strap includes forming the body portion with a first material, forming the termination portion with a second material having a higher rigidity than the first material, and securing the termination portion to a structural member. Additional steps include forming the first material as a plurality of yarn fibers and forming the second material as a rigid fiber composite.
The preferred method includes forming the body portion with spanwise fibers, forming the termination portion with spanwise fibers and angled fibers, and alternating layers of spanwise fibers with angled fibers to form a termination portion thickness that is greater than the body portion thickness. Additional steps include forming a transition region between the body portion and the termination portion with a third material having a higher rigidity than the first material and a lower rigidity than the second material, forming the length of the transition region as being at least as long as the width of the strap, and forming the third material from a flexible fiber composite including urethane.
This unique method and apparatus provides a strap termination with significantly increased strength, and which can be used in tight packaging areas.